We're launching a search activity when hitting the global search button (i.e. the hardware search key, if present, or a search icon in the Action Bar.) It should be noted here that the search activity is very complex in terms of functionality and a mere search bar expanded into the Action Bar is not sufficient for us. A mockup follows.
Now the question is: what's the proper way to navigate away from this screen? One important thing to mention is that on the search screen, we automatically focus the search field and open the keyboard. Here are the 3 solutions we came up with:
show a cross in the upper left corner of the action bar, as is used to close an active action mode. This is what our designer first came up with and triggered this discussion. We initially argued against it, since we only ever saw this icon being used by Android to navigate away from an active action mode. However, see my other points to see why this may still be a good case for it.
Use the Up button. This is what I originally had in mind. However, we quickly decided that this is not a good solution, since this would mean that when reaching this global search screen from e.g. a user profile activity (where we show the Action Bar plus search button too), there is no logical parent except for the home screen, which would make for an awkward user flow. Moreover, the search screen is also used to refine an existing search, so it is more used as an options/settings screen that exists orthogonal to other activities from a user's perspective.
Not show any icon at all and rely on the back button. We initially didn't want to do this, since we always show the keyboard and the user would have to hit back twice to exit out of this screen, which is annoying. Not a game breaker or anything, but not great either.
I initially asked this on Google Plus, here is Roman Nurik's response:
Generally you shouldn't launch a separate activity right away and instead expand a SearchView in the action bar. Up should collapse the action back into an icon. If however you absolutely must launch a new activity (e.g. if search is more than just plaintext search), Up leading to the top-level home screen or whatever else is the structural parent of Search seems like the choice best-aligned with our design guidelines.